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Image compression: Lossless and lossy compression

JPEG

Shooting with a reduced number of pixels is not the only way to produce smaller image files. It is possible to shoot at high resolution and then remove some of the data before the file is saved. This is called image compression. The data is restored when the file is opened up on a computer, returning the image to its original file size and, hopefully, resolution.

There are various compression systems available. The one used by Canon digital cameras is called JPEG (pronounced jay-peg). In simple terms, the compression software looks for areas where all the pixels have a similar colour and brightness. It records the full data for the first pixel and then writes the digital equivalent of ‘ditto’ for matching adjacent pixels. This takes up less space than recording full details for each pixel.

When the image file is opened for viewing, the software uses the data from the first pixel to recreate the additional pixels.

Different degrees of JPEG compression are possible. At maximum compression levels, the file size can be as little as 1/100th of the size of the original file. However, as the compression increases, the software starts to batch pixels that are less close in colour and brightness. When the file is opened again, the recreated pixels are less close to their original brightness and colour values. This leads to loss of detail.

It is important to realise that once a file is compressed, some of the image data is lost and cannot be recovered. As with resolution settings, you should never use a compression level that takes the file below the quality you need in the final image.

Also, data is lost each time a modified file is compressed. If you open a compressed file, make changes and then re-save it as a JPEG, additional data will disappear.

Lossless and lossy compression/Canon RAW format

File compression techniques that lose some of the original data are called ‘lossy’. However, ‘lossless’ compression is also available. This is used by Canon digital cameras when the highest resolution image is stored as RAW data.

Lossless compression uses mathematical algorithms to pack all the data into less space. This compression is fully reversible, so that when the file is opened, all the data is still there. This means that there is no reduction in image quality.

Lossless compression cannot achieve the file size reductions offered by JPEG compression, but Canon RAW format files can often be saved at a quarter of the size of an uncompressed file. (The actual file size is affected by the subject and the ISO speed.)

The RAW setting captures all the sensor data and records it without any significant processing inside the camera. This means that you can work on the file once it has been downloaded to a computer, experimenting with different settings until you get the image you want. This extends to adjusting the colour balance and other image parameters such as sharpness and contrast.

A RAW file is often described as a ‘digital negative’, because it offers the creativity given by a film negative in the darkroom.

Disk transfer

Although the Canon RAW image format provides maximum resolution, you need to run the file through compatible software to see the image on a computer screen.

However, if you save the image to your CompactFlash card as a compressed JPEG file, you can open the image using a wide range of computer imaging applications. Various modifications can be made to the JPEG file, though for extensive changes, it is better to work with a RAW image. Every time a JPEG file is opened, modified and saved, image data is lost. This is not the case when using RAW files.

File sizes

You can set four resolutions and two compression levels on the EOS-1D Mark II and EOS-1Ds Mark II, giving five different file sizes.

EOS-1D Mark II N

Image

Pixels

Compression

File size

RAW

3504 x 2336

lossless

7.9Mb

Large

3504 x 2336

fine

3.2Mb

Medium 1

3104 x 2072

fine

2.6Mb

Medium 2

2544 x 1696

fine

1.9Mb

Small

1728 x 1152

fine

1.1Mb

EOS-1Ds Mark II

Resolution

Pixel recorded

Megapixels

Suggested print size

RAW

4992 x 3328

lossless

14.6Mb

Large

4992 x 3328

fine

5.5Mb

Medium 1

3600 x 2400

fine

3.2Mb

Medium 2

3072 x 2048

fine

2.6Mb

Small

2496 x 1664

fine

1.9Mb

Which is best?

If you want the maximum image quality from your digital camera, all the images should be shot at maximum resolution with no JPEG compression (RAW). However, this not only takes up lots of CompactFlash card space, but also increases the amount of time needed to store the image. This time factor will not worry some photographers, but is important if you want to shoot rapid sequences at sports events.

On the other hand, if you want rapid access to your images, using JPEG compression allows you to read the files directly from the CompactFlash card.

The great thing about digital cameras is that there are no film and processing costs, which reduces the cost of experimenting. Shoot the same subject at different combinations of resolution and compression and take a look at the images on screen. Print images from the smallest and largest files and see if there is a noticeable difference. You might be surprised to discover that reducing resolution has less effect on the images than you imagine.